Radio telecommunications systems using code division multiple access (CDMA) transmit multiple channels simultaneously in the same frequency band.
As shown in FIG. 1, a baseband signal 2 for transmission, which as used herein includes digital signals which may have been processed by a processor 4 to compress them, is modulated by a modulator 6 using a modulation scheme such as Quaternary phase shift keying (QPSK) or quadrature amplitude modulation (QAM) so as to define a sequence of “symbols” that are to be transmitted.
The symbols have both an in phase and imaginary component, and occur at a rate known as a symbol rate. The symbols from the modulator undergo two further processing operations prior to transmission.
The symbols are spread by a spreading code 8 so as to spread the data from each symbol. The spread data is then further multiplied by a scramble code 10 which is specific to the cell that the mobile or base station is operating in. The result of these processes is to generate “chips” which are then transmitted follow up-conversion 12 to the desired transmit frequency.
The spreading codes are selected such that they make the symbols mutually orthogonal. This condition applies whilst all of the chips are in time alignment (which is easy to achieve at the transmitter) but the occurrence of multiple transmission paths in the propagation channel between the transmitter and receiver can result in multiple versions of the same transmit sequence of chips arriving with different time delays and amplitudes at the receiver, as shown in FIG. 2.
One method of recovering the transmitted data is to use a rake receiver.
A rake receiver is schematically shown in FIG. 3. It comprises a plurality of individual processing channels 30-1, 30-2 to 30-N, known as fingers. Each finger allows the relative time alignment between the received signal and a despreading/de-scrambling code to be adjusted. This enables signal power from each significant transmission path to be recovered and brought into time alignment at a combiner.
Within the rake receiver, the correlators within each finger act to correlate the received real and imaginary data from the radio frequency front end against the scrambling code and the spreading code thereby undoing the coding used at the transmitter. Each finger contains several correlators.
In transmit mode diversity systems having time multiplexed pilot signals it is known to modify one of the correlators within at least one of the fingers of the rake to include two further correlators for correlating the time multiplexed pilot (TMP) signals from antenna A and antenna B. The additional correlators are provided “down stream” of the correlators used to process incoming chips.
FIG. 4 illustrates a prior art topology used within one correlation path (of which there are several) in a rake finger. It should be noted that each finger correlates against early, on time, and late versions of a channel pilot signal and also against several potential data streams. Thus the arrangement of FIG. 4 can be regarded as a single processing channel within a finger and there will be several such channels within any given one of the fingers within the rake receiver. The channel comprises three correlators, numbered 30, 32 and 34. Each correlator has the same internal construction. Each correlator works in a known manner to form a running total of the result of multiplying an input signal to the correlator by a descrambling code, despreading code or time multiplexed pilot derotate code, as appropriate, and to output the result of the running total over each symbol length for each code.
In practice two time multiplexed pilot correlators 32 and 34 are provided in a single one of the channels of the rake receiver. The other channels within the rake receiver do not need to handle the time multiplexed pilot derotation and hence they can be simplified by omission of the correlators 32 and 34.